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Biological Decision Making: The Strange Connection Between Cows and Bacteria


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Cow Relaxing, photo by Richard Gailey

Are cows more likely to lie down the longer they stand? This apparently simple question turns out to have an unexpected answer. The study, by a team of Scottish sustainable livestock systems researchers, won the 2013 IgNobel Prize for probability and has left many people puzzled about the mysteries of cow behavior.

Unlike horses, cows lie down to rest, and the researchers predicted that the longer the cows were standing, the more tired they would be and the more likely they would be to lie down. Conversely, cows that had rested adequately, they predicted, would be more likely to stand up the longer they had been lying down. Their hypotheses were pretty straightforward and their results potentially useful — understanding the cows’ preferences for standing and resting times would help in scheduling feeding and milking in large farms.

Using a wireless device attached to the cows’ legs, the team tracked nearly 60,000 cow “lying episodes,” collecting data on how long each cow was lying down or standing up before she decided to switch. But while the cows were more likely to stand up the longer they had been lying down, the reverse didn’t hold true; there was no correlation between how long a cow had been standing and how likely she was to lay down. The researchers concluded that understanding the whims and desires of cows will require more research.

***

Understanding the whims and desires of bacteria is the focus of significant research in microbiology. Some strains of bacteria, for example, can either swim around in search of food or stop moving and save energy. Bacteria, of course, have no brains, so the “decision” they make to swim or sit still depends on the action of only a handful of genes and proteins in each cell. These proteins respond to levels of food and water available in the environment, allowing bacteria to conserve energy in times of stress.

But bacteria are so small that the chaotic motion of the their proteins and DNA bumping into each other is enough to randomly flip the genetic switches controlling whether the cell swims or not. Even in a population of genetically identical bacterial cells living in a nutrient rich environment, there will be some cells that have randomly decided to stop swimming.

Bacillus subtilis cells swimming or sitting still

In a population of identical swimming cells (green), some will randomly choose to sit still (red). Microscope video of Bacillus subtilis cells courtesy of Nate Lord.

Johan Paulsson’s lab at the Harvard Medical School Department of Systems Biology studies this biological randomness. Researchers in the lab use complex mathematical models and precise observations of the behavior of single cells to understand the underlying chaos of biology. In a recent paper in the journal Nature, members of the Paulsson lab studied how randomness plays a role in how the soil bacterium Bacillus subtilis decides to swim or sit still.

To keep track of single cells, the team created a device that immobilizes bacteria along tracks wide enough to fit only one cell at a time. As the cells grow, they divide in half, pushing down along the channel until they eventually fall out of the bottom. With a microscope, the researchers could watch thousands of cells over hundreds of generations.

Video of Bacillus subtilis cells in the measurement device, courtesy of Nate Lord.

Cells trapped in these channels couldn’t swim away, so in order to see which state the cells were in the team had to genetically engineer the cells to flash different colors as they switched back and forth between “swim” and “stop” states. Swimmers were labeled green while the non-swimmers were labeled red. By timing how long each cell stayed red or green before they switched, the researchers could see just how random the switch was. Would the cells want to “relax” the longer they had been swimming? Would they start swimming again after they had “rested” long enough?

As they expected, the researchers found that the switch from swimming to resting was random — the cells could swim for just a few generations or hundreds. But for the switch in the opposite direction the cells stayed still for much more precise periods of time. The longer a cell was resting, the more likely it was to move again. Both bacteria and cows, it seems, will choose when to rest at random, while timing how long they stay still.

***

There’s something both deeply satisfying and profoundly confusing in such a weird coincidence, providing both the “aha!” and the “huh?” that drive scientists to keep asking new questions. What links the behavior of cows and bacteria? Is there some deep biological truth underlying how both of these organisms choose when to rest?

In Bacillus, the switch between swimming and resting is controlled by just three genes. The behavior of these genes depends on their DNA sequence and the ways that they interact, the conditions in the environment, the cell’s history — how long it was resting — and a fundamental randomness in gene expression.

Cow behavior, on the other hand, is much more complicated, controlled by the action of many genes and many cells — neural networks, muscles, and nerves — the conditions in the environment — is it sunny or shady? is there food over there? — the behavior of nearby cows, the animal’s history and preferences and, it seems, a little bit of randomness.

The mechanisms controlling the random resting behavior of cows and bacteria are certainly different, but the coincidence does show something that is perhaps universal about the complexity of biology. In all organisms, from the smallest bacteria to the largest mammals, biological behavior emerges from the interaction of genes, proteins, and cells with environmental conditions and the organism’s history, in ways that are often hard to predict. Regardless of scale, biology is a mix of nature, nurture, and randomness.

Christina Agapakis About the Author: Christina Agapakis is a biological designer who blogs about biology, engineering, engineering biology, and biologically inspired engineering. Follow on Twitter @thisischristina.

The views expressed are those of the author and are not necessarily those of Scientific American.





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